Alkali hydrides in org. synthesis

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Discussion Overview

The discussion centers on the reactivity of alkali hydrides in organic synthesis, specifically examining the reaction of a ketone with potassium hydride (KH) and the acidity of hydrogen atoms in relation to carbonyl groups. Participants explore the conditions under which such reactions occur and the implications of using different types of hydrocarbons as reagents.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant proposes a reaction involving C2H5COCH3 and KH, noting the acidity of the hydrogen atoms beta to the carbonyl group.
  • Another participant challenges the feasibility of the reaction if an alkane like C4H10 were used instead, citing the significantly higher pKa of alkanes compared to hydrides.
  • A third participant corrects the terminology used regarding hydrogen atoms, clarifying that the hydrogens in question are alpha hydrogens, not beta hydrogens, and explains the role of resonance stabilization in the acidity of these hydrogens.
  • This participant also suggests a method for evaluating reactivity by considering the stabilization of resulting structures after bond breaking.

Areas of Agreement / Disagreement

Participants express differing views on the terminology and the feasibility of reactions involving alkanes versus alkali hydrides. There is no consensus on the specific outcomes of the proposed reactions, and the discussion remains unresolved regarding the conditions under which these reactions would occur.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the acidity of hydrogen atoms and the definitions of alpha and beta hydrogens. The implications of resonance stabilization are also noted but not fully explored.

Who May Find This Useful

Readers interested in organic chemistry, particularly those studying reaction mechanisms and the properties of alkali hydrides in synthesis, may find this discussion relevant.

espen180
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I am interested in reactions of the type

C2H5COCH3 + KH -> C2H5COCH2K + H2

In this paticular reaction, I understand that the participating hydrogen is beta to the carbonyl group since the oxygen's electronegativity makes the beta-hydrogen atoms acidic, and KH acts like a base. However, would the reaction take place had the reagent been an alkane, for example C4H10? If so, at what carbon atom will the reaction most likely take place?
 
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Nope. pKa for hydride is something like 28-29 whereas that of an alkane is around 50.

Sodium hydride is sold as a solid coated with a hydrocarbon oil for safety purposes.
 
espen: watch your terminology. We only describe carbons as alpha, beta, gamma, etc. with respect to a carbonyl. As such, those are alpha hydrogens, NOT beta hydrogens.

Also, to expand on your understanding a bit more: the reason those alpha hydrogen are more acidic than the corresponding alkane is not only due to inductive electron withdrawal by the carbonyl group. If the alpha carbon were deprotonated, there would be resonance stabilization of this anion (you can draw this out for yourself to see). This is important in the haloform reaction.

One of the best ways that I've found to approach questions of reactivity in organic chemistry is to imagine a bond breaking, then asking, "What features of this molecule would stabilize this resulting structure?" This gets to the heart of some chemical principles.
 
Thanks for the help!
 

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